The present invention relates to a semiconductor device and a method of forming the semiconductor device, and more particularly, to a nonvolatile memory device and a method of forming the nonvolatile memory device.
Nonvolatile memory devices retain data when power is removed from the system. One type of nonvolatile memory device is a flash memory device used for electrically recording and erasing date. In a conventional flash memory device, a unit cell includes a tunnel oxide layer, a floating gate, an oxide-nitride-oxide (ONO) layer, and a control gate electrode that are sequentially stacked. The flash memory cell can store one-bit data (logic 0 or 1) by electrically charging or discharging the floating gate.
In the flash memory device, electrons should be kept in the floating gate for a long time for good data retention characteristics of the flash memory device, and the endurance of the tunnel oxide layer should be good for normal functioning of the flash memory device after a number of recording and erasing operations.
According to a conventional method for improving the data retention characteristics of a flash memory device, the thickness of the tunnel oxide layer is increased. In this case, the threshold voltage of the flash memory cell increases, and thus the data retention characteristics of the flash memory device can be improved. Due to iterative recording and erasing operations, traps can be present in the tunnel oxide layer. The traps can capture electrons tunneling the tunnel oxide layer during the recording or erasing operation. This can degrade the endurance of the tunnel oxide layer. The possibility of trapping in the tunnel oxide layer increases in proportion to the thickness of the tunnel oxide layer. That is, when the thickness of the tunnel oxide layer is increased for improving the data retention characteristics of the flash memory cell, the endurance of the tunnel oxide layer can be degraded since the number of traps increases. In particular, as the number of traps increases, the heat-resistive characteristics (heat stress characteristics) of the flash memory device can be degraded. The heat stress characteristics are related to an upper temperature limit above which the flash memory device cannot normally operate. For example, when the flash memory device is heated to a high temperature (thermally stressed), electrons captured by the traps are released to the outside of the tunnel oxide layer (i.e., the more traps the more released electrons), changing the threshold voltage of the flash memory cell. As a result, the characteristics of the flash memory device are degraded.